Atoms, Elements, and Minerals

Physical Geology 12/e, Chapter 2

Minerals

• A mineral is a naturally occurring, inorganic, crystalline solid, which is physically and chemically distinctive.

• Form in the geosphere (most minerals), hydrosphere (e.g., halite), biosphere (e.g., calcite), and even the atmosphere (e.g., water ice, as snow)

• Consistent and recognizable physical and chemical properties

Atoms and Elements• An element is a substance that can not be broken

down to other substances by ordinary chemical reactions

• An atom is the smallest unit of a substance that retains the properties of that element – Composed of 3 types of subatomic

particles• Protons (positively charged)

• Neutrons (zero net charge)

• Electrons (negatively charged)

• A molecule is the smallest unit of a compound that retains the properties of that substance

Atomic Structure

• Protons and neutrons form the nucleus of an atom– Represents tiny fraction of the volume at the

center of an atom, but nearly all of the mass

• Electrons orbit the nucleus in discrete shells or energy levels– Shells represent nearly all of the volume of

an atom, but only a tiny fraction of the mass

– Numbers of electrons and protons are equal in a neutral atom

– Ordinary chemical reactions involve only outermost shell (valence) electrons

Isotopes• Atoms of an element with different

numbers of neutrons are called isotopes• Isotopes may be either stable or unstable

– Stable isotopes retain all of their protons and neutrons through time

– Unstable or radioactive isotopes spontaneously lose proton(s) and/or neutron(s) from their nuclei over time

• Stable isotopes of oxygen can be used to track climate change over time– How? (Explain) Warm vs. Cold….

Chemical Bonding• Ionic bonding

– Involves transfer of valence electrons from one atom to another

• Covalent bonding– Involves sharing of valence

electrons among adjacent atoms

– Example: diamond & graphite

• Metallic bonding– Electrons flow freely throughout

metals; results in high electrical conductivity

Ionic bonding of NaCl (sodium chloride)

Composition of Earth’s Crust

• Common elements– Nearly 97% of the atoms in Earth’s

crust are represented by the 8 most common elements

• O, Si, Al, Fe, Ca, Na, K, Mg

• Common mineral types– Most minerals are silicates (contain Si

and O bonded together)

• Minerals have crystalline structures– Regular 3-D arrangement of atoms

Insert Box 2.3 - Fig. 2A here

Silicate Structures• The Silicon-Oxygen tetrahedron

– Strongly bonded silicate ion

– Basic structure for silicate minerals

• Sharing of O atoms in tetrahedra– The more shared O atoms per tetrahedron,

the more complex the silicate structure• Isolated tetrahedra (none shared)

– olivine

• Chain silicates (2 shared)– pyroxenes

• Double-chain silicates (alternating 2 and 3 shared)

– amphibole

• Sheet silicates (3 shared)– mica

• Framework silicates (4 shared)– Quartz & feldspars

Non-silicate Minerals• Carbonates

– Contain CO3 in their structures (e.g., calcite - CaCO3)

• Sulfates– Contain SO4 in their structures (e.g., gypsum - CaSO4

. 2H2O)

• Sulfides– Contain S (but no O) in their structures (e.g., pyrite - FeS2)

• Oxides– Contain O, but not bonded to Si, C or S (e.g., hematite - Fe2O3)

• Native elements– Composed entirely of one element (e.g., diamond - C; gold - Au)

Minerals• A mineral must meet the following criteria:

– Crystalline solid• Atoms are arranged in a consistent and orderly geometric pattern

– Forms through natural geological processes– Has a specific chemical composition

• May include some internal compositional variation, such as the solid solution of Ca and Na in plagioclase)

• Rock-forming minerals– Although over 4000 minerals have been identified, only a few hundred

are common enough to be generally important to geology (rock-forming minerals)

– Over 90% of Earth’s crust is composed of minerals from only 5 groups (feldspars, pyroxenes, amphiboles, micas, quartz)

Mineral Properties

• Color– Visible hue of a mineral

• Streak– Color left behind when mineral is

scraped on unglazed porcelain

• Luster– Manner in which light reflects off

surface of a mineral

• Hardness– Scratch-resistance

• Crystal form– External geometric form

• Physical and chemical properties of minerals are closely linked to their atomic structures and compositions

Mineral Properties

• Cleavage– Breakage along flat planes

• Fracture– Irregular breakage

• Specific gravity– Density relative to that of water

• Magnetism– Attracted to magnet

• Chemical reaction– Calcite fizzes in dilute HCl

Luster

• Metallic - M– Chrome car parts, or

rusty iron

• Nonmetallic - NM– Glassy (vitreous) = windows

or porcelain

– Earthy = unglazed pottery

– Uncommon NM Luster: • resinous

• silky

• pearly

Mohs’ Hardness Scale

• Testing Procedure:– Fingernail = 2.5

– Penny = 3.5

– Iron Nail = 4.5

– Glass = 5.5

– Steel File = 6.5

• Mineral Hardness will be given as a range:

2.5 - 3.5

End of Chapter 2